Viruses

Intro to Viruses Movie

I. General info:

A. Viruses are pathogens (cause disease) that affect organisms in all 6 kingdoms

B. Don’t belong to any kingdom because they are not considered living organisms

C. Acellular (non-living)

II. Why are viruses acellular?

A. Don’t grow or carry on metabolism

B. Do not come directly from other viruses

C. Lack cytoplasm and organellesD. Have only 1 type of nucleic acid

(DNA or RNA); cells have bothE. Have only a few enzymesF. Can survive crystallization

DRAW Fig. 19-11, pg. 483

III. Viral Characteristics

A. All viruses are small; only visible through electron microscope

B. Consists of two parts:1. Nucleic acid core - can be

either DNA or RNA, but never both

2. Capsid - outer protein coat

Nucleic Acid Core and Capsid

IV. Capsid

A. Determines the shape of the virus

B. Allows the virus to recognize, bind to and enter specific cells

C. Highly specificD. 3 Basic Capsid Shapes:

1. Geometric shape (e.g. polyhedral)

Ex. Adenovirus (respiratory virus) and HIV

Adenovirus

HIV virus

Reverse transcriptase

Protein coat

membraneRNA

Receptors

2. Helical or rod-shaped (ex. tobacco mosaic virus)

3. Combined shape (ex. bacteriophage)

T4 Bacteriophage Tobacco MosaicVirus

Influenza Virus

Head

Tail sheath

DNA

Tail fiber

RNACapsid

Surfaceproteins Membrane

envelope

RNA

Capsidproteins

DRAW FIG 19-9; pg. 479

V. Viral Infection

A. A virus can reproduce only by infecting living cells

B. Once inside, they use the cell’s machinery to replicate and produce more viruses

C. This happens in one of two ways:

1. Lytic Infection – virus enters the cell, makes copies of itself, then kills the cell by causing it to burst

a. Attachment - virus attaches itself to the receptor site of host cell

b. Injection - virus use enzymes to enter the cell wall of host, then injects its nucleic acid

c. Replication - viral nucleic acid is replicated by the cell’s machinery and produces proteins that form the viral enzymes and capsids

1) Retroviruses have a special enzyme called reverse transcriptase*.

*This enzyme converts RNA to DNA before replication begins.

d. Assembly - Viral parts are assembled to form new complete viral units

e. Lysis - New viruses break out of host cell, killing it and attacking new cells

2. Lysogenic Infectiona. Attachment and Infection - same as

lytic

b. Splicing - The viral DNA is spliced into the DNA of the host.

c. Cellular duplication - During normal cell division the viral DNA is also duplicated and passed on with the cell’s own DNA

d. Activation - An environmental stress stimulates the activation of the viral DNA and stages 3-5 of the lytic cycle begin (ex. cold sore outbreak)

DRAW FIGURE 19-10 on page 481

VI. Harmful EffectsA. Viral diseases in human

1. Viruses cause disease by disrupting an organism’s homeostasis

2. Examples: Rabies, mumps, measles, AIDS, herpes (chickenpox & shingles), influenza (some), polio, common cold, conjuctivitis (pink eye), warts, smallpox, hepatitis, mononucleosis, HPV, some cancers & many more diseases

Mumps – infect saliva gland cells

Measles – infect nasal and throat cells

Shingles - infects nerve cells

Polio – infects nerves that control muscles

“Tree Man”HPV

B. Viral diseases in livestock1. Foot-and-mouth disease, Rous

sarcoma, West Nile, avian influenza (bird flu)

C. Viroids and Prions (virus-like particles)1. Viroids are single-stranded RNA

molecules w/no capsid that cause disease in plants

a. Disrupt metabolism and stunt plant growth

2. Prions “protein infectious particles” contain no DNA or RNA; cause disease in animalsa. Abnormally folded proteinsb. Cause the formation of protein clumps

called amyloidsc. Ex: BSA (mad cow disease), scrapie

(sheep) and Creutzfeldt-Jakob disease (Kuru people)

VII. Beneficial EffectsA. Can be used to insert desirable

genetic information into cells.1. Genetically engineered organisms

like tulip coloration2. Deliver correct genes into human

cells (cure for cystic fibrosis?)B. Possible antibiotic (kills bacteria)

VIII. Combating Viral Infections

A. Antiviral Drugs 1. Work by interfering with

virus replication stage2. These are not cures

B. VaccinesC. Organism’s immune system